Applicant hereby claims priority under 35 U.S.C. xc2xa7119 to Japanese Application No. 2002-001957, filed Jan. 9, 2002, titled Low-Voltage Drive Circuit and Method for the Same, the disclosure of which is incorporated herein by reference.
The present invention relates to circuits for electromagnetic-induction pointing devices, and more specifically, to a low-voltage drive circuit for driving a sensor coil in an electromagnetic-induction coordinate input device at a constant current.
For discharging a current to a sensor coil using a single power supply in low-voltage drive circuits in conventional electromagnetic-induction coordinate pointing devices, a common potential of sensor coils must be shifted to a midpoint potential. A reference circuit with low impedance is required to avoid noise. Such reference circuits have a complex circuit design. Furthermore, such reference circuits do not efficiently. utilize electric power. The drive circuits also include a complementary constant-current circuit unit, which has a drive source circuit segment and a drive sink circuit segment. Each of the segments requires a bias voltage of approximately 1 V. If a single 3 V power supply is used, sufficient drive voltage of the sensor coils is not achieved.
A conventional low-voltage drive circuit for driving a sensor coil in an electromagnetic-induction coordinate input device at a constant current is best shown in FIG. 3. An AC signal is input to an input terminal IN5. A PNP transistor Tr20 and a differential amplifier 30 are each connected to a power supply Vcc with a resistor R20 therebetween. The PNP transistor Tr20 and the differential amplifier 30 function as a drive source circuit segment. Based on the input AC signal, the differential amplifier 30 and the PNP transistor Tr20 output a constant-current AC signal to one end of a sensor coil 10. Also, an AC signal is input to an input terminal IN6. An NPN transistor Tr21 and a differential amplifier 32 are each connected to ground with a resistor R21 therebetween. The NPN transistor Tr21 and the differential amplifier 32 function as a drive sink circuit segment. Based on the input AC signal, the differential amplifier 32 and the NPN transistor Tr21 output a constant-current AC signal to one end of the sensor coil 10. The PNP transistor Tr20 and the NPN transistor Tr21 are complementarily connected to each other, and combine the constant-current AC signals corresponding to the signals input from the input terminals IN5 and IN6 into one signal, which is then output.
The other end of the sensor coil 10 is connected to a reference circuit unit 34, so that a common potential of the sensor coil 10 is shifted to a midpoint potential. Also, the other end of the sensor coil 10 is grounded with a capacitor C20 therebetween. The reference circuit unit 34 is needed for AC driving the sensor coil 10. Thus, the size of the circuit is increased, and thus current consumption in the circuit increases.
When the sensor coil 10 is AC driven, the midpoint of the sensor coil 10 has half the voltage of the power supply Vcc by using the reference circuit 34, thus restricting a drive voltage range.
Accordingly, it is an object of the present invention to provide a drive circuit that reduces the size of the overall drive circuit, thereby reducing current consumption by driving a sensor coil at a constant current without using a reference circuit. The drive circuit has a wide driving voltage range by biasing a drive source circuit segment and a drive sink circuit segment separately.
A low-voltage drive circuit for driving a sensor coil in a coordinate input device at a constant current comprises: a constant-current output drive circuit unit; a DC coupling capacitor provided between an output of the constant-current output drive circuit unit and the sensor coil; and a constant-current output bias circuit unit for controlling a DC bias voltage of the output of the constant-current output drive circuit unit to be a midpoint potential of an operating voltage range for the constant-current output drive circuit unit. The midpoint potential is a target value of control. The vicinity of the midpoint potential, as a result of control, is also regarded as the midpoint potential, wherein the vicinity range is determined by a conventional method well known in the art. (The same applies to the following embodiments of the present invention.) The constant-current output drive circuit unit preferably includes a drive source circuit segment and a drive sink circuit segment, which function as a complementary constant-current circuit unit.
In a low-voltage drive circuit for driving a sensor coil in a coordinate input device at a constant current according to a second embodiment of the present invention, a constant-current output drive source circuit segment and a constant-current output drive sink circuit segment function as a complementary constant-current circuit unit. An output of the drive source circuit segment is connected to the sensor coil with a first DC coupling capacitor therebetween. An output of the drive sink circuit segment is connected to the sensor coil with a second DC coupling capacitor therebetween. The output of the drive source circuit segment is connected to a first constant-current output bias circuit unit, and the output of the drive sink circuit segment is connected to a second constant-current output bias circuit unit. The first and second constant-current output bias circuit units control DC bias voltages of the outputs of the drive source circuit segment and drive sink circuit segment, respectively, to be midpoint potentials of the respective operating voltage ranges of the drive source circuit segment and the drive sink circuit segment.
The first constant-current output bias circuit unit mentioned above may include a differential amplifier, a power supply, an amplifier, a resistor, a capacitor, and an NPN transistor (a bipolar transistor), or an FET or the like which corresponds to the bipolar transistor. In the drive source circuit segment, for example, a differential amplifier and a PNP transistor (a bipolar transistor) or an FET or the like which corresponds to the bipolar transistor convert an input AC signal to a constant-current AC signal that drives the sensor coil at a constant current via the DC coupling capacitor.
The second constant-current output bias circuit unit may include a differential amplifier, a power supply, an amplifier, a resistor, a capacitor, and a PNP transistor (a bipolar transistor), or an FET or the like which corresponds to the bipolar transistor. In the drive sink circuit segment, a differential amplifier and an NPN transistor (a bipolar transistor), or an FET or the like which corresponds to the bipolar transistor, may convert an input AC signal to a constant-current AC signal that drives the sensor coil at a constant current via the DC coupling capacitor.
A low-voltage drive method for driving a sensor coil in a coordinate input device at a constant current comprises the following steps: providing a constant-current output drive circuit unit; connecting a DC coupling capacitor between an output of the constant-current output drive circuit unit and the sensor coil; and controlling a DC bias voltage of the output of the constant-current output drive circuit unit to be a midpoint potential of an operating voltage range for the constant-current output drive circuit unit by a constant-current output bias circuit unit.
A low-voltage drive method for driving a sensor coil in a coordinate input device at a constant current according to the second embodiment of the present invention includes the steps of providing a constant-current output drive source circuit segment; connecting a first DC coupling capacitor between an output of the drive source circuit segment and the sensor coil; controlling a DC bias voltage of the output of the drive source circuit segment to be a midpoint potential of an operating voltage range for the drive source circuit segment by a first constant-current output bias circuit unit; providing a constant-current output drive sink circuit segment, the drive sink circuit segment functioning as a complementary constant-current circuit unit together with the drive source circuit segment; connecting a second DC coupling capacitor between an output of the drive sink circuit segment and the sensor coil; and controlling a DC bias voltage of the output of the drive sink circuit segment to be a midpoint potential of an operating voltage range for the drive sink circuit segment by a second constant-current output bias circuit unit.
The constant-current output bias circuit unit is connected to the output of the drive circuit unit. A bias voltage can thereby be maintained at a midpoint potential of a voltage range in which the drive circuit unit can be driven, even if the coupling capacitor is provided between the drive circuit unit and the sensor coil. Consequently, the sensor coil can be driven at a constant current without a reference circuit, thus reducing the size of the circuit. Thus, the current consumption in the circuit may be reduced.
In particular, in the second embodiment of the present invention, the drive source circuit segment and the drive sink circuit segment are connected to respective constant-current output bias circuit units, thereby allowing drive voltage ranges to be extended.